I do not have a Google checkout account myself... but it looks like one can use it to get "unlimited" (until 12/31/06) cash back from cash back credit cards by "selling" stuff to himself or to relatives/friends. Google offers free credit card processing via Google checkout until 12/31/06.
Google promo link
Here's how this promotion works:
* From November 8 through December 31, 2006, you can process all Checkout transactions for free, even if you aren't an AdWords advertiser. If you're already an AdWords advertiser, you can process your Checkout transactions for free regardless of your AdWords spend.
* You don't need to take any action to be eligible for this promotion. You can simply process your sales through Checkout as usual.
* Any valid Checkout order you receive during the promotion will qualify for free processing. (Orders you've received before 11/8 won't qualify, regardless of when you charge them.)
* There's no limit to the number of sales you can process for free during this promotion.
Starting January 1, 2007, the standard transaction fee will apply again. If applicable, your regular free transaction processing (based on your December spend) will resume.